doc: update "Testing and CI" in Collaborator Guide

Update "Testing and CI" in the Collaborator Guide. Remove redundant
material. Shorten sentences. Remove incorrect material. (Specifically,
we don't require test cases to be included in all pull requests that
modify exectuable code. For example, if code is refactored, then passing
existing tests is sufficient.)

PR-URL: nodejs#24884
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

COLLABORATOR_GUIDE.md

@@ -174,20 +174,17 @@ the comment anyway to avoid any doubt.
 
 ### Testing and CI
 
-All bugfixes require a test case which demonstrates the defect. The
-test should *fail* before the change, and *pass* after the change.
-
-All pull requests that modify executable code should also include a test case
-and must be subjected to continuous integration tests on the
-[project CI server](https://ci.nodejs.org/). The pull request should have a CI
-status indicator.
-
-Do not land any Pull Requests without passing (green or yellow) CI runs. If you
-believe any failed (red or grey) CI sub-tasks are unrelated to the change in the
-Pull Request, use "Resume Build" in the left navigation of the relevant
-`node-test-pull-request` job. It will create a new `node-test-pull-request` run
-that preserves all the green results from the current job but re-runs everything
-else.
+All fixes must have a test case which demonstrates the defect. The test should
+fail before the change, and pass after the change.
+
+All pull requests must pass continuous integration tests on the
+[project CI server](https://ci.nodejs.org/).
+
+Do not land any Pull Requests without passing (green or yellow) CI runs. If
+there are CI failures unrelated to the change in the Pull Request, try "Resume
+Build". It is in the left navigation of the relevant `node-test-pull-request`
+job. It will preserve all the green results from the current job but re-run
+everything else.
 
 #### Useful CI Jobs